Piezoelectric sensor and acceleration sensor

ABSTRACT

A piezoelectric sensor has a piezoelectric element having a specific axis of sensitivity and a package for the piezoelectric element. The package has a rectangular parrallelopiped configuration with opposite longitudinal end surfaces having a height-to-width ratio of about 1:1. External lead electrodes are formed to cover at least the longitudinal end surfaces. A method and apparatus for detecting if the sensor is disposed in the proper posture is also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a piezoelectric sensor for sensingacceleration or vibration and, more particularly, to a piezoelectricsensor to be mounted on a substrate such as a printed circuit board.

2. Description of the Related Art

An acceleration sensor which is a kind of piezoelectric sensor isusually used by packaging it or mounting it on a mounting substrate suchas a circuit board which has a circuit for processing accelerationsignals generated by the sensor.

Mounting of an acceleration sensor on a mounting substrate has causedtwo major problems in the prior art.

The first problem is as follows. The design of an acceleration sensor tobe mounted on a substrate is made such that the sensor sensesacceleration acting in directions perpendicular to the plane of themounting substrate or acceleration acting parallel to the plane of themounting substrate, depending on factors such as the use of the sensorand specifications of the circuit board. It is therefore necessary thatthe acceleration sensor be mounted on the circuit board so as to besensitive to acceleration acting in either one of the above-mentioneddirections.

Meanwhile, the acceleration sensor has a rectangular parallelopipedconfiguration having six surfaces, and is adapted to be jointed to thecircuit board at a specific one of the six surfaces or a surfaceopposite to this surface. This is because the acceleration sensor has aflattened rectangular parallelopiped configuration having a rectangularbottom and top surfaces and side surfaces of a height which is small ascompared with the two sides of the rectangle comprising the top andbottom surfaces, so that the sensor can be soldered to the circuit boardwhile being stably held on the circuit board. For instance, the heightranges from 0.5 to 0.7 times as large as the length of the shorter sideof the rectangle defining the top and bottom surfaces.

Consequently, it has been necessary to prepare two types of accelerationsensors: an acceleration sensor of the type which is capable of sensingacceleration acting perpendicularly to the plane of the circuit boardand an acceleration sensor of the type which is capable of sensingacceleration acting parallel to the plane of the circuit board. Anacceleration sensor of either one of these two types is selected foruse. This not only raises the costs of production of accelerationsensors but also requires costs for storing and administrating these twotypes of acceleration sensors, resulting in a rise in the costs ofvarious products incorporating such acceleration sensors.

The second problem is as follows: The circuit board on which anacceleration sensor is packaged may be deflected for any reason, beforeor after the packaging of the acceleration sensor. specifically, thedeflection of the circuit board causes a corresponding deformation ofthe acceleration sensor. Such a deformation of the acceleration sensordue to an external force adversely affects the piezoelectric memberwhich senses acceleration, thus hampering sensing of acceleration. Thisproblem is particularly serious when the acceleration sensor is intendedto sense acceleration acting perpendicularly to the plane of themounting substrate, because in such a case the direction of accelerationto be sensed coincides with the direction of deflection of the mountingsubstrate. Consequently, the acceleration sensor may be influenced bythe deflection of the circuit board so as to erroneously produce anacceleration signal even when there is no acceleration acting on thesensor.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide apiezoelectric sensor which can reduce adverse effects due to thedeflection of the substrate and a piezoelectric sensor which can bemounted on a circuit board selectively either in a direction to senseacceleration perpendicular to the plane of the circuit board or in adirection to sense acceleration parallel to the plane of the circuitboard, thereby obviating at least one of the two major problemsencountered with the prior art.

To this end, according to one aspect of the present invention, there isprovided a piezoelectric sensor comprising: a piezoelectric elementhaving a specific axis of sensitivity; and a package in which thepiezoelectric element is packaged; wherein the package has a rectangularparallelopiped configuration with end surfaces having a height-to-widthratio approximating 1:1, and wherein external lead electrodes are formedon at least the end surfaces.

According to a second aspect of the present invention, there is providedan acceleration sensor comprising: a bimorph element having an axis ofhighest sensitivity extending in a direction which substantiallycoincides with a line normal to the plane of a circuit board; and a caseassembly for fixing and supporting both longitudinal ends of the bimorphelement, the case assembly being adapted to be mounted on the circuitboard at both its longitudinal ends which support both longitudinal endsof the bimorph element; wherein the bimorph element has a pair ofpiezoelectric ceramic plates each having a signal electrode and anintermediate electrode formed on the opposite major surfaces thereof,the piezoelectric ceramic plates being joined to each other face to faceat their surfaces having the intermediate electrodes such that theintermediate electrodes are coupled to each other; each of thepiezoelectric ceramic plates being sectioned in the longitudinaldirection of the bimorph element into three sections including a centralsection and both end sections at border lines which are positioned suchthat when the bimorph element is deformed in response to deflection ofthe circuit board, the quantity of charges generated in the centralsection equals the sum of the quantities of the charges generated inboth the end sections, the central section and both end sections of eachpiezoelectric ceramic plates being polarized thicknesswise of thepiezoelectric ceramic plate in opposite directions, the directions ofpolarization of the central section and both end sections of one of thepiezoelectric ceramic plates being opposite to those of the other of thepiezoelectric ceramic plates.

According to a third aspect of the present invention, there is providedan electronic part having a polyhedral body on opposing end surfaces ofwhich are formed electrodes for outputting voltages of differentpolarities, the electronic part comprising: a conductive film formed onone of the surfaces of said body orthogonal to the end surfaces havingthe electrodes at a predetermined position closer to one of the endsurfaces than to the other, the conductive film having an area largeenough to be simultaneously contacted by a pair of probe terminals forapplying a voltage.

According to a fourth aspect of the present invention, there is provideda method of examining posture of an electronic part which has to beplaced in a predetermined posture in terms of up and down, left andright and front and back directions, the method comprising: bringing apair of probe terminals into contact with a potion of an upwardlydirected surface of the electronic part disposed in an examinationposition in an arbitrary posture, the portion being closer to one of theend surfaces having electrodes than to the other; applying a voltagebetween the probe terminals; and determining, based on the presence orabsence of electrical current between the probe terminals, whether theelectronic part as the examination object has been placed in a correctposture.

According to a fifth aspect of the invention, there is provided anapparatus for examining posture of an electronic part and which has tobe placed in a predetermined posture in terms of up and down, left andright and front and back directions, the apparatus comprising: a pair ofprobe terminals adapted to be brought into contact with a potion of anupwardly directed surface of the electronic part disposed in anexamination position in an arbitrary posture, the portion being closerto one of the end surfaces having electrodes than to the other;detecting means for applying a voltage between the probe terminals andfor detecting presence or absence of electrical current between the pairof probe terminals; and determining means for determining, based on theresults of the detection by the detecting means, whether the electronicpart as the examination object has been placed in a correct posture.

The piezoelectric sensor in accordance with the present invention isdesigned to be stably seated on a printed circuit board regardless ofthe posture of mounting thereof on the printed circuit board. Therefore,a single piezoelectric sensor can provide a variety of directions ofaxis sensitivity, by changing the posture of the piezoelectric sensormounted on the printed circuit board, thus widening the adaptability ofthe piezoelectric sensor. The present invention therefore achieves aremarkable reduction in the costs of production and management ofpiezoelectric sensors and, hence, the price of the same.

The acceleration sensor of the present invention offers an advantage inthat, even if the bimorph element is deformed due to the influence ofdeflection of the circuit board on which the acceleration sensor ismounted, charges generated as a result of the deformation are canceledby each other, so that no signal charges are derived from the sensorwhen no acceleration is acting thereon, whereby the influence ofdeflection of the circuit board is eliminated. However, whenacceleration acts on the acceleration sensor, charges are generated inthe central section and both end sections of each piezoelectric ceramicplate, based on the relationship between the directions of polarizationof these sections and the tensile and compression stresses caused by thedeformation. Such charges are picked up as output signal voltage,without being canceled, thus providing a high level of output signal.According to the invention, therefore, it is possible to suppressinfluence caused by deflection of the circuit board, while achieving ahigher degree of reliability. Moreover, a high level of sensor outputcan be obtained when acceleration actually acts on the sensor.

The electronic part in accordance with the present invention enables,with simple electronic processing, confirmation of the posture of theelectronic part which is to be mounted on a printed circuit board or tobe loaded in a tape carrier correctly in a predetermined posture, with ahigh degree of accuracy while avoiding increase in production cost.

The examination method and apparatus of the present invention enablesthe confirmation of the electronic part under the condition describedabove. In particular, the examination apparatus offers a remarkablereduction in the installation cost as compared with the conventionalexamination system which relies upon image processing techniques.

The above and other objects, features and advantages of the presentinvention will become clear from the following description of thepreferred embodiments when the same is read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partly-sectioned perspective view of an embodiment of theacceleration sensor in accordance with the present invention;

FIG. 2 is an exploded perspective view of the acceleration sensor asshown in FIG. 1;

FIG. 3 is a schematic illustration of a piezoelectric element deformedunder action of acceleration;

FIGS. 4(a) and 4(b) are illustrations of the postures in which theacceleration sensor of FIG. 1 is mounted on a printed circuit board;

FIG. 5 is an illustration of the acceleration sensor of FIG. 1 carriedby a tape carrier;

FIGS. 6(a) to 6(d) are perspective views of a second embodiment of theacceleration sensor in accordance with the present invention, showingpositioning of the sensor relative to probe terminals;

FIG. 7 is a block diagram schematically showing the construction of anexamining device used in combination with the acceleration sensor of thesecond embodiment;

FIG. 8 is a perspective view of a modification of the accelerationsensor;

FIG. 9 is a perspective view of another modification of the accelerationsensor;

FIG. 10 is a perspective view of still another modification of theacceleration sensor;

FIG. 11 is an illustration of an acceleration sensor carried by a tapecarrier;

FIG. 12 is a schematic sectional perspective view of the thirdembodiment of the acceleration sensor;

FIG. 13 is an illustration of a bimorph element deformed under action ofan acceleration; and

FIG. 14 is a schematic. illustration of an acceleration sensor deformeddue to deflection of a circuit board.

DESCRIPTION OF PREFERRED EMBODIMENTS

First Embodiment

A first embodiment of a piezoelectric sensor of the present invention,serving as an acceleration sensor sensitive to acceleration, will bedescribed with specific reference to FIGS. 1 to 5. FIG. 1 is apartly-sectioned perspective view of an embodiment of the accelerationsensor in accordance with the present invention. FIG. 2 is an explodedperspective view of the acceleration sensor as shown in FIG. 1. FIG. 3is a schematic illustration of a piezoelectric element deformed underaction of acceleration. FIGS. 4(a) and 4(b) are illustrations of thepostures in which the acceleration sensor is mounted on a printedcircuit board. FIG. 5 is an illustration a tape carrier carrying theacceleration sensor.

Referring to these Figures, an embodiment of the acceleration sensor 1has a piezoelectric element 2 of a type known as a "bimorphpiezoelectric element", a package 3 and external leads 4a, 4b. Theacceleration sensor 1 has a sensitivity axis which, as indicated by anarrow, extends in the direction of thickness according to a definitiongiven later of the piezoelectric element 2.

The piezoelectric element 2 has a pair of strip-shaped piezoelectricceramic plates 5, 5 each having on its major surfaces a signal pickupelectrode 6 and an intermediate electrode 7, the piezoelectric ceramicplates 5, 5 being jointed face to face at their intermediate electrodes7, 7. The direction of stacking of the pair of piezoelectric ceramicplates 5, 5 is referred to as the direction of thickness of thepiezoelectric element 2. Each of the piezoelectric ceramic plates 5, 5is divided in the longitudinal direction into three sections 5a, 5b and5b at border lines L, L. The arrangement is such that a tensile stressor a compression stress acts on each section 5a, 5b, 5b in response toacceleration G acting on the sensor. The central sections 5a and bothend sections 5b of both piezoelectric ceramic plates 5, 5 are polarizedin opposite directions along the thicknesses of the piezoelectricceramic plates 5, as indicated by arrows A, B and C, D. Morespecifically, the central sections 5a, 5a are polarized towards eachother as indicated by the arrows A and C, whereas both end sections 5b,5b; 5b, 5b are polarized away from each other, as indicated by thearrows B and D.

The package 3 has a pair of clamping members 8, 9 each having asubstantially square-bottomed U-like form when viewed from one sidethereof, for clamping each longitudinal end portion of the piezoelectricelement 2 from the upper and lower sides of the piezoelectric element 2.The package 3 also has a pair of cover members 10, 11 each having asubstantially squarebottomed U-shaped cross-section, the cover members10, 11 being secured to the left and right ends of the clamping members8, 9 so as to cover the left and right side faces of the piezoelectricelement 2. The package 3 generally has a rectangular parallelopipedconfiguration, with opposing end surfaces having a height-to-width ratioof 1:1 i.e., a square configuration. Representing the height and thewidth of the end surface by L1 and L2 (FIG. 1), respectively, accordingto the present invention, it is preferred that the ratio L1/L2 is set toabout 1.0, i.e., to meet the condition of L1=L2, although the inventiondoes not exclude such a height-to-width ratio L1/L2 of from about 0.9 to1.1.

In accordance with the present invention, the external lead electrodes4a, 4b are provided in the form of layers laid on the longitudinal endsurfaces of the package 3 including the longitudinal end surfaces of thepiezoelectric element 2. One of the external lead electrodes 4a, 4b isconnected to one of the signal pickup electrodes 6, 6 formed on thepiezoelectric ceramic plates 5, 5, while the other of the external leadelectrodes 4a, 4b is connected to the other of the signal pickupelectrodes 6, 6.

The operation of the described acceleration sensor 1 is as follows. Itis assumed that the central portion of the piezoelectric element 2 ismainly deflected to be convex upward as illustrated in FIG. 3, inresponse to an acceleration G. In such a case, positive (+) charges aregenerated on the outer surface of the central section 5a of thepiezoelectric ceramic plate 5 which is on the outer side as viewed inthe direction of deflection of the piezoelectric element 2, based on therelationship between the direction A of polarization and the tensilestress Pt. Similarly, positive charges are generated on the outersurfaces of both end sections 5b, 5b of the same piezoelectric ceramicplate 5, due to the relationship between the direction B of polarizationand the compression stress Pc.

The positive charges generated on the outer major surfaces of thecentral section 5a and both end sections 5b, 5b of the above-mentionedpiezoelectric ceramic plate 5 are delivered from the signal pickupelectrode 6 to the associated external lead electrode 4b while beingsummed with each other.

In the meantime, negative (-) charges are generated on the outer surfaceof the central section 5a of the piezoelectric ceramic plate 5 which ison the inner side as viewed in the direction of deflection of thepiezoelectric element 2, based on the relationship between the directionC of polarization and the Compression stress Pc. Similarly, negativecharges are generated on the outer surfaces of both end sections 5b, 5bof the same piezoelectric ceramic plate 5, due to the relationshipbetween the direction D of polarization and the compression stress Pt.These negative charges are transferred to external lead electrodes 4afrom the signal pickup electrode 6 associated with this piezoelectricceramic plate 5.

Although negative charges and positive charges are generated on theinner surfaces of the piezoelectric ceramic plates 5, 5, respectively,the negative charges and the positive charges cancel each other via theintermediate electrodes 7, 7.

A description will now be given as to why tension and compression areapplied to the piezoelectric ceramic plates 5, 5 of the piezoelectricelement 2 in response to the action of the acceleration G. When thewhole acceleration sensor 1 is accelerated by the acceleration G, suchan acceleration directly acts on the package 3 tending to move thepackage 3 in the direction of the acceleration G. However, thepiezoelectric element 2 is not directly subjected to such anacceleration so that the piezoelectric element 2 tends to remain in thestate before the application of the acceleration. Namely, inertia forceis generated in response to the acceleration G so as to act on thepiezoelectric element 2. The end sections 5b, 5b of both piezoelectricceramic plates 5, 5 tend to move together with the package 3 by beingpulled by the package 3, whereas the central sections 5a, 5a tend toremain at their original position, whereby the central portion of thepiezoelectric element 2 is mainly deformed to deflect in the directionopposite to that of action of the acceleration G, such as to be convexupward in the illustrated case. As a consequence, tensile stress Pt isgenerated in the central section 5a of the piezoelectric ceramic plate 5which is on the outer side of the deflection, i.e., the upperpiezoelectric ceramic plate 5, while compression stress Pc is generatedin each of the end sections 5b of the same piezoelectric ceramic plate5, as will be seen from FIG. 3. Conversely, the piezoelectric ceramicsheet 5 which is on the inner side of the deflection, i.e., the lowerpiezoelectric ceramic plate 5, is stressed such that compression stressPc appears in the central section 5a, while tensile stress Pt appears ineach of the end sections 5b, 5b of this piezoelectric ceramic plate 5.

The acceleration sensor 1 which has been described is mounted on aprinted circuit board 12 such that one of the four major surfaces isdirected upward, depending on the direction of sensitivity axisaccording to the design requirement, in a manner as shown in FIG. 4(a)or a manner as shown in FIG. 4(b). Thus, the direction of thesensitivity axis is determined by the posture of the piezoelectricsensor 1 on the circuit board. Turning sideways or other movement of theacceleration sensor 1 during soldering due to lack of stability isavoided regardless of the posture of the piezoelectric sensor. In FIGS.4(a) and 4(b), numerals 1a and 1b denote upper and lower surfaces, whilenumerals 1c and 1d represent right and left side surfaces. When thepiezoelectric sensor 1 is mounted in a manner shown in FIG. 4(a), theaxis of sensitivity extends in the direction parallel to the printedcircuit board 12 as indicated by the arrow, whereas, when the same ismounted in the posture as shown in FIG. 4(b), the axis of sensitivityextends in the direction perpendicular to the plane of the printedcircuit board 12.

A process for packaging the acceleration sensor 1 typically employs atape carrier 20. FIG. 5 shows such a tape carrier 20 by way of example.The tape carrier 20 is adapted to hold a plurality of independentacceleration sensors 1 of the type described above. More specifically,the tape carrier 20 has an embossed tape 21 having substantially squarerecesses 23 approximating the configuration of the acceleration sensor1, at a predetermined pitch along the length thereof, and an upper tape22 which is bonded to the upper side of the embossed tape 21 so as toclose the above-mentioned recesses 23. All the acceleration sensors 1 onthe same tape carrier 20 are disposed in the same posture in regard tothe axis of sensitivity as indicated by arrows, depending on thedirection of sensitivity axis to be obtained when these sensors 1 aremounted on circuit boards or the like. For instance, the posture inwhich the acceleration sensors 1 are accommodated in the recesses 23 ofthe tape carrier 20 is determined such that the left side surface id ofeach acceleration sensor 1 is exposed through an opening of the recess23 in the tape carrier 20.

Preparation of a plurality of acceleration sensors on the tape carrier20 in the described manner facilitates packaging of the sensors 1 on theprinted circuit boards. Namely, the packaging can be performed by asimple process having the steps of peeling the upper tape 22 off thetape carrier 20 so as to expose one surface of the acceleration sensor 1in each recess 23, picking up the acceleration sensor 1 by, for example,a vacuum sucker (not shown) acting on the exposed surface of theacceleration sensor 1, and placing the acceleration sensor 1 on thetarget position on the printed circuit board.

The described embodiment is only illustrative and various changes andmodifications may be made thereto. For instance, the piezoelectricelement 2 may be a so-called uni-morph type element which iscantilevered by the clamping members 8, 9 at its one end, rather thanbeing supported at both its longitudinal ends as in the describedembodiment. Furthermore, the piezoelectric element 2 may be constructedto serve as a vibration sensor, although an acceleration sensor has beenspecifically mentioned.

In the illustrated embodiment, each external lead electrode 4 is formedto cover not only each end surface of the package but also the portionof the peripheral surface of the package near each longitudinal end ofthe package 3. The external lead electrode 4, however, may be formed tocover only each end surface of the package 3.

Furthermore, the directions of polarizations of the sections of thepiezoelectric ceramic plates 5 may be determined such that thedirections A and C of polarizations of the central sections 5a, 5a areaway from each other, while the directions B and D of polarizations ofthe end sections 5a, 5a; 5a, 5a are towards each other.

Second Embodiment

The acceleration sensor 1 described as the first embodiment of thepresent invention has a square cross-section perpendicular to thelongitudinal axis. It is therefore impossible to discriminate thedirection of the sensitivity axis based on the appearance. This poses arisk that the acceleration sensor 1 when mounted on a printed circuitboard, for example, is placed in a wrong posture. Such a risk can beovercome when the acceleration sensor 1 of the first embodiment ismounted on the circuit board in accordance with a method which will bedescribed hereinunder. With this method, it is possible to mount theacceleration sensor 1 such that the sensitivity axis is extended in thedesired direction without fail, so that the advantage of theacceleration sensor, which resides in alternative posture of mounting,can be fully enjoyed.

Although the acceleration sensor of the first embodiment is specificallymentioned in the following description, it is to be noted that thefollowing method can be carried out by using other types of electronicparts. FIGS. 6(a) to 6(d) are perspective views of the accelerationsensor, while FIG. 7 is a block diagram schematically showing theconstruction of an acceleration sensor examining device.

Although not shown, the acceleration sensor 1 has a piezoelectricelement arranged in a manner like a bridge and packaged in an insulatingpackage which has a rectangular parallelopiped configuration. Thedirection of the axis of sensitivity varies as indicated by arrowsdepending on the posture of the piezoelectric element in the sensor.When the acceleration sensor is placed in the posture as shown in FIG.6(a), the axis of sensitivity extends in the horizontal direction.

External lead electrodes 32, 33 for delivering voltages of oppositepolarity, i.e., positive and negative voltages, are provided in the formof layers laid on both longitudinal ends of the acceleration sensor 1.More particularly, the external lead electrode 32 is laid on the frontend surface 31a of the acceleration sensor 1 and the regions of theupper, lower, left and right surfaces 31c to 31f adjacent to the frontend surface 31a of the sensor 1. Likewise, the external lead electrode33 is laid on the rear end surface 31b and the regions of the upper,lower, left and right surfaces 31c to 31f adjacent to the rear endsurface 31b of the sensor 1. More specifically, the external leadelectrode 32a on the front end surface 31a delivers a voltage ofpositive polarity, while the external lead electrode 33 on the rear endsurface 31b delivers a voltage of negative polarity. A dummy electrode34 formed of a web-like conductive film is provided on the upper surface31c among the surfaces 31c to 31f, at a predetermined position closer tothe positive external lead electrode 32 than to the negative externallead electrode 33. The dummy electrode 34 extends over the entire widthof the upper surface 31c and has a size large enough to allow a pair ofprobe terminals 36 to contact therewith for the purpose of applying avoltage which will be described later. The dummy electrode 34 serves asa sign indicating that the acceleration sensor 1 should be mounted on,for example, a printed circuit board (not shown) in such a posture thatthe surface in which the dummy electrode 34 is present is directedupward, and the surface on which this dummy electrode is provided isdetermined depending on factors such as the direction of the sensitivityaxis, polarities of the external lead electrodes 32, 33, and so forth.The dummy electrode 34 may be formed, for example, by firing withsilver, application of a conductive paste or by plating.

Whether the acceleration sensor 1 has been situated in correct postureis examined before the sensor 1 is packaged on, for example, a printedcircuit board. The examination is conducted by using an examinationdevice 35 which will now.be described with reference to FIG. 7.

The examination device 35 has a pair of probe terminals 36 for applyinga voltage, locating/actuating device 37 for locating the probe terminals36 both in vertical and horizontal directions, voltage applicationcircuit 38 for applying a predetermined voltage to the probe terminal36, detector 39 for detecting any electrical current between the probeterminals 36, discriminator 40 for discriminating whether or not theacceleration sensor 1 is in correct posture, based on the results ofdetection performed by the detector 39, display 41 for displaying theresults of the discrimination performed by the discriminator 40, andcontrol circuit 42 for controlling the operations of the describedcomponents in order to execute the examination processing. The detector39, which is in this embodiment a current-sensitive detector, may be ofthe type which senses resistance or capacitance. When such a detector isused, the discriminator 40 performs the discrimination based on theresults of comparison between the detected value and a predeterminedreference value.

Using the examination device 35 having the described construction, anexamination as to whether the acceleration sensor 1 is in correctposture or not is examined in accordance with the following procedure.

The acceleration sensor 1 as the examination object is placed in apredetermined posture in regard to up and down, left and right and frontand back directions. When the posture of the acceleration sensor 1 iscorrect, the upper surface 31c, i.e., the surface carrying the dummyelectrode 34, is directed upward and, when the sensor 1 in such aposture is viewed in top plan view, the front end surface 31a isdirected to the south and the rear end surface 31b is directed to thenorth, with the left and right side surfaces 31e and 31f respectivelydirected to the east and west. Thus, the dummy electrode 34 ispositioned closer to the south end of the sensor than to the north end.

The probe electrodes 36 are brought into contact with the accelerationsensor 1. Namely, the locating/actuating device 37 is activated to movethe probe electrodes 36 to an area where the acceleration sensor 31 isdisposed. More specifically, the probe electrodes 36 are moved to andlocated at a position where the dummy electrode 34 should exist when theacceleration sensor 31 is disposed in the correct posture, and arefurther moved into contact with the dummy electrode 34.

Then, a predetermined voltage is applied between the probe electrodes 36across the sensor 1 so as to detect any electrical current flowingbetween the probe electrodes 36.

If the acceleration sensor 1 is in the correct posture as shown in FIG.6(a), the probe electrodes 36 safely contact with the dummy electrode 34so that an electrical current flows between the probe electrodes 36through the dummy electrode 34 so as to be detected by the detector 39.In response to the output from the detector 39, the discriminator 40determines that the acceleration sensor 1 is in the correct posture, andthe control circuit 42 gives instruction to the display 41 to cause thelatter to display that the examined acceleration sensor 1 is in thecorrect posture.

However, if the acceleration sensor 1 is in a wrong posture, e.g.,turned upside down, falls sideways or rotated such that the front endsurface is directed improperly, as shown in FIGS. 6(b) to 6(d), noelectrical current flows between the probe electrodes 35 because theseelectrodes do not contact with the dummy electrode 34 on theacceleration sensor 1. When no electrical current is detected by thedetector 39, the discriminator 40 determines that the accelerationsensor 1 is in a wrong posture, so that the control circuit 42 givesinstruction to cause the display 41 to display that the examinedacceleration sensor 1 is in the wrong posture.

The operator monitors the display on the display 41 to check whethereach acceleration sensor is in the correct posture. When theacceleration sensor has been placed in the correct posture, the assemblyprocess advances to the next step for soldering, otherwise the processskips to a step in which an operation is performed to correct theposture of the acceleration sensor 1. Although in the describedembodiment, display 41 is employed to display the results of theexamination, such a display operation is not essential. Namely, one ofthe above-mentioned two steps may be automatically selected inaccordance with the output from the discriminator.

Thus, the examination device 35 is required only to perform a two-stepoperation applying a voltage through the probe terminals 36 anddiscriminating presence or absence of electrical current flowing betweenthe probe terminals 36. Thus, the described examination device 35handles much less amount of data as compared with known examinationapparatus which rely on image processing techniques, whereby the costsof the facility for the examination are greatly reduced. In addition,examination is performed with a higher degree of reliability than bymanual inspection and makes it possible to realize an inexpensiveexamination system incorporated in an automatic production line.

The dummy electrode 34 on the acceleration sensor 1 can be formedsimultaneously with the formation of the external lead electrode 32, 33,without requiring any additional step in the production process, so thatcost of production of the acceleration sensor 1 is not substantiallyraised.

The described embodiments are only illustrative and various changes andmodifications may be made thereto.

Firstly, it is to be noted that the square cross-section of theacceleration sensor is not essential and other cross-sectional shapesalso may be employed. For instance, acceleration sensors asmodifications which will now be described have a rectangularcross-section perpendicular to the longitudinal axis.

In these modifications, the positive external lead electrode 32 and thenegative external lead electrode 33 provided on the acceleration sensor1 are made to have different areas as shown in FIGS. 8 to 10 so as toprovide information as to the posture, instead of the dummy electrode 34used in the described embodiments.

More specifically, FIG. 8 shows a modification in which the portion ofthe positive external lead electrode 32 on the upper surface 31c of theacceleration sensor 1 is extended to a region contactable with the probeelectrodes 36, whereas the size and area of the portions of the externallead electrode 32 on the lower surface 31d and left and right sidesurface 31e, 31f, as well as the size and area of the negative externallead electrode 33 on all the surfaces 31c, 31d, 31e and 31f, are sodesigned that these portions are not extended to regions contactablewith the probe electrodes 36.

In the modification shown in FIG. 9, the portion of the external leadelectrode 32 on the upper surface 31c is extended to the same length asthat in the first embodiment, but the portions of this electrode 32 onthe lower surface 31d and left and right side surfaces 31e, 31f, as wellas the portions of the negative external lead electrode 33 on all thesurfaces 31c, 31d, 31e and 31f, are retracted so as not to be contactedby the probe terminals 36.

In the modification shown in FIG. 10, the portion of the positiveexternal lead electrode 32 on the upper surface 31c of the accelerationsensor 1 has the same size and area as that in the embodiment shown inFIG. 9, whereas the portions of the positive and negative external leadelectrodes 32, 33 on both side surfaces 31e, 31f are omitted. Theportion of the positive external lead electrode 32 on the lower surface31d, as well as the portions of the negative external lead electrodes 33on the upper and lower surfaces 31c, 31d, is formed to extend over onlyhalf the width of the acceleration sensor 1 so as not to besimultaneously contacted by two probe electrodes 36.

Each of the modifications shown in FIGS. 8 to 10 can be examined byusing the examination device 35 described before. Examinations of themodifications shown in FIGS. 9 and 10, however, require that thepositions at which the probe terminals 36 contact the accelerationsensor are shifted from that in the cases shown in FIGS. 6(a)-6(d) and8.

Although in the described embodiments the external lead electrodes 32,33 are formed to cover not only the longitudinal front and rear endsurfaces 31a, 31b but also portions of the upper, lower, left and rightsurfaces 31c to 31f adjacent to the respective end surfaces, the presentinvention does not exclude such an arrangement that the external leadelectrodes 32, 33 are formed to cover only the front and rear endsurfaces 31a, 31b.

In the embodiment described in connection with FIGS. 6(a) to 10, theexamination device 35 is used for the purpose of examining the postureof the acceleration sensor 1 in the course of mounting of the sensor ona printed circuit board. The same examination device 35 also can be usedfor the purpose of examining the posture of the acceleration sensor 1when the sensor 1 is loaded on a tape carrier 50 as shown in FIG. 11which is used, as explained before, for the purpose of conveying aplurality of acceleration sensors 1.

The tape carrier 50 is adapted to hold a plurality of independentacceleration sensors 1 of the type described above. More specifically,the tape carrier 50 has an embossed tape 51 having substantially squarerecesses 53 approximating the configuration of the acceleration sensor1, at a predetermined pitch along the length thereof, and an upper tape52 which is bonded to the upper side of the embossed tape 51 so as toclose the above-mentioned recesses 53. All the acceleration sensors 1 onthe same tape carrier 50 are disposed in the same posture in regard tothe axis of sensitivity, depending on the direction of sensitivity axisto be obtained when these sensors 1 are mounted on a circuit boards orthe like. For instance, the posture in which the acceleration sensors 1are accommodated in the recesses 53 of the tape carrier 50 is determinedsuch that the upper surface 31c of each acceleration sensor 1 is exposedthrough the opening of the recess 23 in the tape carrier 50. Loading ofthe acceleration sensors 1 in the predetermined posture on the tapecarrier 50 can be executed advantageously and effectively by using theexamination device 35 which is capable of ascertaining that theacceleration sensors 1 have been placed in the correct posture.

Third Embodiment

A third embodiment of the acceleration sensor in accordance with thepresent invention will now be described with reference FIG. 12, which isa schematic perspective view of a third embodiment, FIG. 13 which is aschematic illustration of deformation of a bimorph element underacceleration, and FIG. 14 which is a schematic illustration of theacceleration sensor deformed due to deflection of a circuit board.

The third embodiment of the acceleration sensor is adapted to bepackaged on the surface of a circuit board 7 after being correctlylocated. The acceleration sensor has a bimorph element 61 having ahighest sensitivity axis S extending in a direction substantially normalto the surface of the circuit board, a pair of end case members 72 whichsupport both longitudinal end portions of the bimorph element 61 byclamping these ends at the upper and lower surfaces of the bimorphelement 61, and a pair of side case members 77 which is integrated withthe end case members 72 while sealing both longitudinal side surfaces ofthe bimorph element 61.

The bimorph element 61 has a pair of strip-like piezoelectric ceramicplates 64, 64 each having a thin-film signal electrode 62 and athin-film intermediate electrode 63 formed on the opposite majorsurfaces thereof, the piezoelectric ceramic plates 64, 64 being joinedtogether at their surfaces having the intermediate electrodes 63. Eachpiezoelectric ceramic plate 64 is divided into three sections along thelength thereof: namely a central section 64a and both end sections 64b,64b at a pair of border lines L, L. The positions of the border lines L,L are so determined that, when the bimorph element 61 has been deformeddue to a deflection of the circuit board 73, the quantity of the chargesgenerated in the central section 64a equals to the sum of the quantitiesof charges generated in both end sections 64b, 64b. The central section64a and both end sections 64b, 64b have been polarized thicknesswise ofthe ceramic plate 64 in opposite directions as indicated by arrows A andB. Similarly, the other piezoelectric ceramic plate 64 also is sectionedinto a central section 64a and end sections 64b, 64b, and these sectionsare polarized thicknesswise in the directions opposite to those of thecorresponding sections of the first-mentioned piezoelectric ceramicplate 64, as indicated by arrows C and D, respectively. Thus, thecentral sections 64a, 64a of both piezoelectric ceramic plates 64, 64 ofthe bimorph element 61 are polarized inward, i.e., towards each other,as indicated by arrows A and C in FIG. 12, whereas both end sections64b, 64b; 64b, 64b of both piezoelectric ceramic plates 64, 64 arepolarized outward, i.e., away from each other, as indicated by arrows Band D. One of the pair of signal electrodes 62 presented on the outersurfaces of the piezoelectric ceramic plates 64, 64 is connected to anexternal lead terminal 78 which covers one longitudinal end of the caseassembly 72, 77, while the other of the signal electrodes 62 isconnected to an external lead terminal 78 which covers the other end ofthe case assembly 72, 77. The acceleration sensor thus constructedsenses acceleration based on the quantity of charges generated by thebimorph element 61.

Deformation of the bimorph element 61 under acceleration mainly appearsin the central section 64a as schematically shown in FIG. 13. In each ofthe piezoelectric ceramic plates 64, 64 forming the bimorph element 13,tensile stress is generated in the central section 64a, while both endsections 64b, 64b sustain compression stress Pc. However, since thecentral section 64a and both end sections 64b, 64b of each piezoelectricceramic plate 64 are polarized in opposite directions, charges generatedin the central section 64a due to tensile stress Pt and chargedgenerated in both end sections 64b, 64b due to compression stress Pc arenot canceled by each other, so that the bimorph element 61 produces alarge quantity of charges, i.e., a high level of output signal.

More specifically, in the piezoelectric ceramic plate 64 which is on theouter side of the bimorph element 61 as viewed in the direction of thedeflection, i.e., the upper piezoelectric ceramic plate 64, positive (+)charges are generated on the outer major surface of the central section64a based on the relationship between the polarization direction A andthe tensile stress Pt. Positive charges are also generated on the outermajor surfaces of both end sections 64b, 64b of the same piezoelectricceramic plate 64, based on the relationship between the polarizationdirection B and the compression stress Pc. Consequently, the positivecharges generated on the outer major surface of the central section 64aand the positive charges generated on the outer major surfaces of bothend sections 64b, 64b are transferred from the signal electrode 62 tothe associated external lead electrode 78 while being summed with eachother.

In the meantime, negative (-) charges are generated in the piezoelectricceramic plate 64 which is on the inner side of the acceleration sensor 1as viewed in the direction of deflection, i.e., in the lowerpiezoelectric ceramic plate 64. More specifically, negative (-) chargesare generated in the outer major surface of the central section 64a ofthis piezoelectric ceramic plate 64, due to the relationship between thepolarization direction C and the compression stress Pc. Negative chargesare also generated in the outer major surfaces of both end sections 64b,64b of this piezoelectric ceramic plate 64, due to the relationshipbetween the polarization direction D and the tensile stress Pt. Thenegative charges generated on the central section 64a and the negativecharges generated on both end sections 64b, 64b are transferred from thesignal electrode 62 to the associated external lead terminal 68 whileenhancing each other. Consequently, large quantities of positive andnegative charges are generated in the bimorph element 61 so that a highlevel of sensor output is derived from this acceleration sensor.

In each of the piezoelectric ceramic plates 64, 64 under acceleration,charges of polarity opposite to that of the charges produced on theouter major surface are generated on the inner major surface, i.e., onthe surface facing the other piezoelectric ceramic plate 64. Thesecharges on the inner major surfaces of both piezoelectric ceramic plates64, 64 are of opposite polarities and, hence, cancel each other throughthe electrical connection between the intermediate electrodes 63, 63 onthese piezoelectric ceramic plate 64, 64.

FIG. 14 schematically shows the state in which a circuit board 13 onwhich the acceleration sensor is mounted has been deflected or a circuitboard 73 after the mounting of the acceleration sensor is deflected, soas to cause a deformation of the whole bimorph element 61. Note that thedeflection of the circuit board 73 deforms the whole bimorph element 61,while the deformation of the bimorph element 61 under accelerationmainly appears in the central section 64a as schematically shown in FIG.13.

In such a case, tensile stresses Pt are generated in all the sections64a, 64b, 64b of the ceramic plate 64 which is on the outer side of thebimorph element 61 as viewed in the direction of the deflection, whilecompression stresses Pc are generated in all the sections 64a, 64b, 64bof the piezoelectric ceramic plate 64 which is on the inner side of thebimorph element 61 as viewed in the direction of the deflection. As aresult, positive charges and negative charges are generated on the outersurfaces of section 64a and sections 64b, respectively, of thepiezoelectric ceramic plate 64 which is on the outer side of the bimorphelement 61 as viewed in the direction of the deflection. Moreover,negative charges and positive charges are generated on the outersurfaces of section 64a and sections 64b, respectively, of thepiezoelectric ceramic plate 64 which is on the inner side of the bimorphelement 61 as viewed in the direction of the deflection.

As is explained above, since the piezoelectric ceramic plates 64, 64 aredivided into the central sections 64a and the end sections 64b, 64c sothat the amount of the charges generated in the central section 64aequals to the sum of the amount of the charges generated in both endsections 64b, 64b, the charges generated in response to the deformationof the bimorph element 61 cancel each other, so that no electricalsignal is derived from the acceleration sensor based on the deformationof the bimorph element caused by deflection of the circuit board.Accordingly, in the case where the bimorph element 61 deflected due tothe deflected circuit board is also subject to an acceleration, thebimorph element 61 can output signals only in response to theacceleration, and detect the degree of acceleration correctly.

Although the invention has been described through its specific form, itis to be noted that the described embodiment is only illustrative andmay be changed or modified within the scope of the invention which islimited solely by the appended claims.

What is claimed is:
 1. An acceleration sensor comprising:a bimorphelement having an axis of highest sensitivity extending in a directionwhich substantially coincides with a line normal to the plane of acircuit board; and a case assembly for fixing and supporting bothlongitudinal ends of said bimorph element, said case assembly beingadapted to be mounted on said circuit board at both its longitudinalends which are in support of both said longitudinal ends of said bimorphelement; wherein said bimorph element comprises a pair of piezoelectricceramic plates each having a signal electrode and an intermediateelectrode formed on opposite major surfaces thereof, said piezoelectricceramic plates being joined to each other face to face at the surfaceshaving said intermediate electrodes such that said intermediateelectrodes are coupled to each other; each said piezoelectric ceramicplate having sections in the longitudinal direction of said bimorphelement, there being three sections including a central section and anend section at each end of the central section, the sections beingpositioned such that, when said bimorph element is deformed in responseto deflection of said circuit board, a quantity of charges is generatedin said central section equal to a sum of the quantities of chargesgenerated in both said end sections, said central section and both saidend sections of each said piezoelectric ceramic plate being polarizedthicknesswise of said piezoelectric ceramic plate in opposite directionsof polarization, the directions of polarization of said central sectionand both said end sections of one of said piezoelectric ceramic platesbeing opposite to those of the other of said piezoelectric ceramicplates.
 2. A piezoelectric sensor, comprising:a piezoelectric elementhaving a specific axis of sensitivity and detecting an accelerationalong said specific axis; and a package in which said piezoelectricelement is provided; wherein said package has a rectangularparallelopiped configuration comprising four major surfaces with endsurfaces having a height-to-width ratio approximating 1:1, whereinexternal lead electrodes are formed on at least said end surfaces, andwherein said package is mountable on a surface of a circuit board suchthat any one of said four major surfaces may be mounted on the circuitboard; wherein said piezoelectric element comprises a pair ofstrip-shaped piezoelectric ceramic plates each having on its majorsurfaces a signal pickup electrode and an intermediate electrode, andsaid pair of strip-shaped piezoelectric ceramic plates is joined face toface at the intermediate electrodes thereof; and further wherein each ofthe strip-shaped piezoelectric ceramic plates is divided into a centersection and two end sections interposing said center section, and saidcenter section is polarized in a direction opposite to that of said endsections.
 3. A piezoelectric sensor, comprising:a piezoelectric elementhaving a specific axis of sensitivity and detecting an accelerationalong said specific axis; and a package in which said piezoelectricelement is provided; wherein said piezoelectric element has a pair ofstrip-shaped piezoelectric ceramic plates joined to each other, and saidpackage includes a pair of clamping members for clamping at least onelongitudinal end of said piezoelectric element from the upper and lowersides thereof so as to support said piezoelectric element, and a pair ofside cover members secured to both sides of said clamping members so asto cover left and right side surfaces of said piezoelectric element; andwherein said package has a substantially rectangular parallelopipedconfiguration comprising four major surfaces with end surfaces having aheight-to-width ratio approximating 1:1, and external lead electrodesare formed on at least said end surfaces, and wherein said package ismountable on a surface of a circuit board such that any one of said fourmajor surfaces may be mounted on the circuit board; wherein saidpiezoelectric element comprises a pair of strip-shaped piezoelectricceramic plates each having on its major surfaces a signal pickupelectrode and an intermediate electrode, and said pair of strip-shapedpiezoelectric ceramic plates is joined face to face at the intermediateelectrodes thereof; and further wherein each of the strip-shapedpiezoelectric ceramic plates is divided into a center section and twoend sections interposing said center section, and said center section ispolarized in a direction opposite to that of said end sections.